From Junk To Champ: Rebuilding A Swap Meet Holley 850 For Cheap

With new products being slung left and right, if you’ve got deep pockets, by all means, have at it. If you desire the industry’s latest and greatest, you better have the greenbacks to back it up. On the other hand, if you’ve only got a cheeseburger allowance (like us) to play with, then scoring a swap-meet carburetor for less then what the average tire alignment costs — you’ve scored.

We scored this original 1968 850cfm Holley mechanical double pumper at a local swap meet for a deal. There's thousands of these great carburetors sitting on guys' shelves and beneath their workbenches. You can score one of these too just by looking around.

For us, the tried and true method of fueling our mills is the traditional four-barrel carburetor. Holley, a name so synonymous with performance and heritage it’s no wonder most gearheads choose it to top off their builds over the competition.

Yeah, it may seem old-school in the age of laptop tuning, but some of us would rather swing a flat-blade screwdriver to adjust the idle mixtures rather than tote around a friggin’ computer.

The Holley carb was used so dependably over the last bazillion years, it’s no wonder they began showing up on many Detroit-built vehicles from the assembly lines. They work, are simple to operate, and if taken care of, showed years of trouble-free operation. What’s more, they’re still around! Visit any number of your local swap-meets or pick-a-part yards, and you’re bound to stumble across one; like we did!

Once the screws have been removed, we could go ahead and also remove the baseplate. This would give us better access to the fuel bowls and linkages. Note the old and worn baseplate gasket.

For the most part, bolting on a used Holley would suffice for any mild build. Then again, with the availability of a complete rebuild kit, which hovers just over a paltry $40, its cheap insurance.

Most renew kits include all the necessary gaskets, O-rings, pump diaphragm, needle and seat, power valves, instructions and hardware for a complete rebuild. For us, this was a no-brainer. Through Holley’s website, we ordered up a complete rebuild kit (4150-style), metering block (PN 134-66), and four corner idle base plate (PN 112-119).

Don’t let the seemingly endless assembly of intricate linkages, springs, and hardware scare you off. We’ll show you exactly how to disassemble a Holley double-pumper and illustrate what it takes to upgrade to a 4-corner idle system with a new Holley base plate. Best part is most of the Holley lineup disassembly and upgrades are all relative. This means no matter which Holley you prefer, all Holley’s carburetors work the same way, just like our double-pumper.

Once the fuel bowls were removed, we could go ahead and peel away the old gasket. We made sure to remove all the gasket material. Any material left on the surface could compromise the new gaskets seal. Through years of use, separating the metering block from the main body may become a chore in itself. If need be, use a small scraper to pry the two apart.

Years of dust, carbon, and grime had built up on the insides of each venturi. We lightly sanded the inner venturi's with a small-piece of Scotch-Brite until each one looked new and fresh. A smooth inner venturi area will help clean up the air flow entering the carburetor. The cleaning solution will most likely leave a small amount of residue on the body. It's important to remove this film. We found a couple cans of Carb Cleaner at a local Napa Auto Parts store. Carb Cleaner also leaves no films or residue; drying completely.

The Teardown

We’ve got some heavy plans for our junkyard gem to go on a completely new engine build. First, we need to get our gem apart! We’ll assume you’ve already gone ahead and disposed of the fuel in the carb responsibly. Next, we turned the carburetor on its side and began to remove the eight Phillips head screws that attach the original baseplate to the main carb body.

“Soak all metal parts in a good carb cleaner for up to 24 hours, then blow out all the passages with some compressed air.”

To make things even easier, we made sure to get a hold of Holley’s in-house carb-guru; Ricky V. Richter. Richter serves as Holley’s Tech Wizard and he gave us some additional points of interest to make your teardown a little simpler. Richter discussed with us the importance of keeping track of everything that’s taken off the carburetor and to take special notes of where the small components belong. If need be, keep a small dish for the smaller items so they don’t get lost.

Continuing with our disassembly, we moved onto the metering blocks. In order to remove them, we used a little elbow grease. In our case, though, since we’ll be replacing the gaskets, it didn’t concern us if they were damaged. Each Holley rebuild kit includes all new gaskets. The metering block also contains the power valve (primary only), and fuel jets. We’ll get to removing those later on. For now, we’ll focus on removing the years of grime.

Holley’s Tech Tip:
Once torn down, inspect all parts for cracks, stripped threads, and any contamination.

Using a small piece of Scotch-Brite can help to loosen and remove any surface debris. In our case, we had some carbon build up in the venturi’s. The Scotch-Brite also helped to remove the baked on metering block gasket.

It’s also important to remember smoothing out any sharp edges that may be present on the main body (metering block area). Any edges could effect the new gaskets ability to seal correctly. We lightly sanded until the area was smooth.

Richter made sure to also mention to, “Inspect all parts for cracks, stripped threads, and any contamination.” Milling any of the components may be necessary, too. As Richter explained, “If the main body where the metering block is installed is warped more than 0.003-inch, it will need to be machined flat.”

Getting the carburetor completely clean is a small task. While not all of us have a cleaning solution to dip a carburetor body into, a couple cans of Carb Cleaner can do the trick, too. Though, we were lucky enough to have a full supply of Berryman’s Chem-Tool on hand. With the carburetor body completely dismantled, we dipped the entire body into the solution and let it sit.

Richter also expressed concern over replacing all gaskets, diaphragms, O-rings, and any rubber parts. Richter added, “Soak all metal parts in a good carb cleaner for up to 24 hours, then blow out all the passages with some compressed air.” The solution will slowly eat away at an excess carbon or grease build up. It’ll also allow the solution to soak into the circuits of the main body. After some time in the solution, we sprayed the entire body with Carb Cleaner. We also made sure to spray Carb Cleaner directly into each circuit to remove any debris.

The accelerator-pump installs fairly quickly. It's located in the bottom of the primary float bowl. Using a small Philips head screwdriver, we removed the pump housing (careful to not lose the spring). Then, we installed the new (supplied) diaphragm from Holley. Holley has a green and black diaphragm. For us, though, we chose the green version. It's safe to use with E85, and Methanol. On the other hand, the black diaphragm is solely used for gasoline only. Next, using the original hardware, we tightened the accelerator-pump housing back together. Note: The accelerator-pump lever arm must be positioned outward.

Access to the needle and seat are found on the ends of each fuel bowl. They're also simple to remove; only requiring a flat head screwdriver and wrench. Unsure of which needles do what? Viton needles are for all gasoline applications, steel needles are used with alcohol or exotic race fuels, and titanium needles are used for their responsiveness during changes in flow rates, as well as for their excellent sealing capabilities.

The Assembly

While most of the rebuild is spent disassembling the unit and thoroughly cleaning the main components, the assembly is much simpler. After each of the components were cleaned, most were left to drip-dry, while others were hand-dried with a shop-towel (keep lint to a minimum). Once the components were dry, we used some compressed air to remove any dust or extra lint. Now, we were ready to begin assembly our Holley double-pumper. We started off with installing the new accelerator pump along with a new diaphragm (green) from the Holley rebuild kit.

Holley’s Tech Tip:
If any cracks are found, replace that part completely. If you find any stripped threads, simply replace that part or Heli-Coil the stripped threads on that particular part.

Keep in mind, our process for assembly may differ than yours. However, all of these steps should be produced at some point during the installation of the new components. Holley’s rebuild kit also includes new needle and seat assemblies, too. Holley offers several needle and seat assemblies seperately for use as replacements or upgrades. They are available in different sizes and materials to suit a wide range of applications.

For the most part, much of the reassembly is replacing many of the smaller gaskets. For example, each adjustment screw for the fuel bowls retains a gasket. Holley provides all new gaskets in their kit. We made sure to swap out all the old ones for the new Holley units. This also included replacing the thin, metal gaskets for each fuel line fitting.

For the street, power valves are a necessity since most street-thumping is handled at part throttle. The importance comes when the throttle is opened up. Additional fuel is needed for a safe air/fuel ratio. In wide-open throttle situations, vacuum in the intake manifold drops to almost zero. At this point the power valve will open and allow more fuel (along with main fuel jets) to the carburetor. The gaskets inside the block can become weak and brittle with years of use. We made sure to play close attention to the power valve in the primary metering block by swapping out the gasket for a new one from Holley.

Each idle mixture screw has a very small cork gasket, which prevents fuel from spilling out onto your hot manifold. A failure here would be catastrophic. We made sure to replace each one with the new gaskets from Holley. To remove, we simply unscrewed them from the metering blocks.

Some of the gaskets in the Holley rebuild kit are small. It's good practice to complete your work in a well-lit area. Also, keep the parts in the same area or in a small dish to prevent losing and of the pieces.

Removal of the power valve is simple. We used a 1-inch box wrench to loosen and then remove the power valve from the block. Then, using a small flat head screw driver, separated the old gasket. Once out, we swapped in the new gasket. Included in the rebuild kit are Holley idle mixture screw gaskets. They are designed to replace your standard idle mixture screw gaskets and are made of cork. The cork will keep your carburetor leak-free when making adjustments to the idle mixture screw.

Another key step in reassembly is installing the new metering block and fuel bowl gaskets. Years of fuel-soak can wreak havoc on the original gaskets that came installed on the carburetors. The small gaskets on each fuel bowl bolt can become weak, too. Each Holley kit comes with a new set for each bowl. We made sure to clean the surface areas before installing the new gaskets.

With majority of the Holley carburetor rebuilt, we could move onto the beauty of our upgrade; Holley’s four-corner idle system base plate. Since our original double-pumper lacked this fucntion, we’ve gone ahead and upgraded to this unit (PN 112-119). What’s more, it comes completely assembled and ready to bolt on to your current main body.

Plus, each Holley rebuild kit comes with a new base plate gasket, too. At around, $110 it was an easy choice. They are manufactured from high-quality cast aluminum and bolt on easily with no modifications. The Holley replacement throttle base plates include a new throttle shaft assembly, throttle blades, and throttle linkage. If you’ve broken off an ear on your base plate, don’t worry—just pick up one of these Holley replacement throttle base plates to solve the problem.

With the assembly through, we were eager to test out our new double-pumper. However, that’ll have to wait. Although we haven’t had the chance to reap the benefits of our labors, we’ve got the perfect mill in mind. We’re slowly preparing for a 440ci Dart engine assembly. Once the motor is complete, we’ll be sure to throw our newly rebuilt Holley double-pumper up top for a full dyno test and tune session. Stay tuned!

The blue gaskets from Holley are constructed from a fiber, which allows them to be reusable. They also won't tear, stick or fall apart after numerous jet changes at the track.

First, we installed the supplied base plate gasket. Then, we set the new Holley four corner idle base plate down and locked it in with the original hardware. We also made sure to reposition the lever under the linkage arm and even added the vacuum caps to the ports we already knew weren't going to be used.

Not too shabby for a couple hours' worth of work, huh? While the stamped numbers don't reveal where this big boy 850cfm hailed from, be it a parts counter sale or it came from Detroit atop a big block-equipped muscle car; either way, we love how our quickie rebuild came out. Now to put it to the test!

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About the author

Sean Haggai

The former Associate Editor of Chevy High Performance, joins publication Chevy Hardcore, Sean is a true blue Bow Tie guy and a core do-it-yourself technician. If it doesn't run a "mouse motor" or a big rat between fenders, Sean ain't interested.